Carbon monoxide (CO) – a tasteless, colourless and odourless gas – found in the exhaust of vehicles and generators has been dubbed the “silent killer” because excessive inhalation is lethal, poisoning the nervous system and heart.

It is not only a danger to the environment but also highly toxic to human beings.

But surprisingly a new study has discovered that low levels of the poisonous gas can have a narcotic effect that helps city dwellers cope with other harmful environmental factors of an urban environment, such as off-the-chart noise levels.

This finding indicates that CO, in small doses, is a boon to the well being of urbanites, better equipping them to deal with environmental stress.

In the study, Prof. Itzhak Schnell of Tel Aviv University’s Department of Geography and the Human Environment and his fellow researchers asked 36 healthy individuals between the ages of 20 to 40 to spend two days in Tel Aviv, Israel”s busiest city.

The most surprising find of the study, said Prof. Schnell, was in looking at levels of CO that the participants inhaled during their time in the city.

Not only were the levels much lower than the researchers predicted — approximately 1-15 parts per million every half hour — but the presence of the gas appeared to have a narcotic effect on the participants, counteracting the stress caused by noise and crowd density.

The results showed that living in a major city might not have as negative a health impact as the researchers were expecting.

Though participants exhibited rising stress levels throughout the day, CO had a mitigating influence, and extended exposure to the chemical had no lasting effects.

The research has been published in the journal Environmental Monitoring and Assessment.